1. Field of the Invention
The present invention relates to the application of pulse oximetric principles to continuous fetal monitoring and, more particularly, to an oximetric apparatus, inserted subcutaneously into the fetus, capable of measuring the oxygen saturation of arterial hemoglobin as well as fetal heart rate (FHR) and electro-cardiogram (EKG) activity during the labor process.
2. The Prior Art
The concept of fetal monitoring during the labor process is well-known and established in the field of obstetrics. The introduction of various electrode devices which attach to the fetal scalp and record FHR and EKG activity of the fetus provided the initial basis for monitoring. (See U.S. Pat. No. Re 28,990 issued to Hon Oct. 1976.) It soon became evident that FHR and EKG activity did not by itself adequately or accurately reflect fetal well-being during labor.
Subsequently, a technique for evaluating fetal well-being by lacerating the fetal scalp, collecting a blood sample and analyzing this sample for a Ph measurement was developed. It should be noted that pH measurements on interstitial fluid of the scalp were often erroneous when compared to pH measurements on blood samples collected from the scalp.
Measuring the pH value of the fetus was an important assessment of fetal well-being because the pH value reflected the arterial oxygenation of the fetus, the importance of which is self-evident. Although scalp pH measurements are an indispensable tool in fetal assessment, these measurements are taken on an intermittent basis and are an invasive and cumbersome procedure.
Various attempts to develop continuous pH monitoring devices have been either clinically impractical or not feasible. One attempt was a spiral-type device shown in the
U.S. Pat. No. 4,320,764. Another pH probe was disclosed in the Peterson et al. U.S. Pat. No. 4,220,110, and was modified for continuous pH monitoring in the Hockberg et al. U.S. Pat. No. 4,658,825. The spiral probe is attached to the epidermis of the fetus. This probe consisted of an ion-permeable membrane envelope which is enclosed at the ends by a pair of optical fibers. The probe, which contains pH-sensitive dye, changes color in response to the hydrogen ion concentration of the interstitial fluid of the fetal scalp. This color change is optically detected and quantified. In addition, the spiral probe also functions to provide FHR and EKG activity by virtue of its contact with the fetal scalp. It should be noted that fetal pH measurements on interstitial fluid have often been erroneous when compared to blood samples, largely due to the common occurrence of caput or swelling of the fetal head during the labor process. In addition, it should be noted that while this probe may provide FHR and EKG activity and pH measurements (albeit often erroneous values), it cannot, by its design, measure the oxygen saturation of arterial hemoglobin. Other design deficits include an unacceptable and non-sterile method of application and a spiral probe which is twice the diameter of conventional probes, and penetrates to five times the depth in the fetal scalp. This size differential would inevitable increase the trauma to the fetus. Accordingly, because of the considerations cited above, this apparatus has not been successfully commercialized.
The principle of pulse oximetry is based upon positioning a pulsating arterial bed between a light source and a photodetector, as shown in the Wilber U.S. Pat. No. 4,394,572. The light source emits wavelengths of red and infrared light which correspond to absorption peaks of oxyhemoglobin (oxygen saturated hemoglobin) and deoxyhemoglobin (unsaturated hemoglobin) in red blood cells entering the arterial capillaries during systole. A background absorption occurs from the hemoglobin remaining in small vessels during diasystole. By utilizing the pulsatile nature of arterial blood, the oximeter is able to separate out the absorption measurements eliminating the measurements of static or non-pulsatile components, namely, venous blood and other tissue components. By rapidly alternating the wavelengths of light transmitted through the tissue, the difference in absorption for total hemoglobin and oxyhemoglobin can be measured for each pulse of arterial capillary blood. An estimated percentage of oxygenated hemoglobin in each pulse can then be calculated from the difference in absorption.
In this way, a precise beat-to-beat calculation of arterial hemoglobin oxygen saturation (AHOS) can be obtained without interference from surrounding venous blood or other tissue components.
The concept of applying the principles embodied within pulse oximetry to fetal monitoring have long posed a number of well-known difficulties, including the inaccessibility of the fetal head and fetal parts, the clinical necessity for monitoring the fetus during early labor with minimal cervical dilation, and attaining sufficient stability in its application to avoid dislodging during labor.
Other prior art attempts to solve these past difficulties are as follows.
The Missanelli et al. U.S. Pat. No. 5,193,542 attempts to perform oximetric fetal monitoring using an apparatus in a T-shaped configuration which consists of a pair of interfacing jaws that clamp onto fetal scalp tissue. The upper jaw houses a pair of LEDs (light source) and the lower jaw houses a pair of photodetectors.
This prior art apparatus has not been used successfully in a clinical setting to date, as evidenced by the fact that there are no published reports of its use available. This device will be difficult at best to apply to the smooth oval-shaped fetal head. Moreover, the constant twisting and moving which occurs by mother and fetus during labor will result in frequent slipping and dislodging of the device. To compound the problems of application and stability, the fetal head (the vertex presentation overwhelmingly predominates in obstetrics at 96%) is often covered in fetal hair and vernix, a white, oily cheese-like material which covers the fetus in utero, as well as other bodily fluids such as blood (maternal), amniotic fluid, fetal meconium, etc. These materials will not only hamper the application, but may result in inaccurate measurements. It should also be noted that this device does not provide for EKG activity. In addition, although precise dimensions are not given by Missanelli, it is difficult to believe that this device, if applied, could be done so before active labor is established.
The Lombardi U.S. Pat. No. 4,501,276 discloses a fetal electrode apparatus. The apparatus consists of an insertion device, such as a hollow tube with a semi-rigid form, and a fetal electrode assembly. The electrode assembly has a piston member such that the electrodes within the insertion device may be pushed out to pierce the fetal head when the insertion device comes in contact with the fetus. The electrodes are designed with beveled edges for piercing the fetal head without having to drill a hole separately. Furthermore, the electrodes do not incorporate using light means for monitoring the fetus.
The Murphy U.S. Pat. No. 4,149,528 discloses an electrode assembly for sensing heart activity, which consists of a guide tube assembly (again a hollow plastic-like tube) with a spiral retaining coil electrode disposed at one end and a handle at the other. By rotating the handle, the spiral retaining coil, which moves on a threaded track, emerges from the opposite end of the guide tube and engages the fetal scalp. A second electrode for monitoring is also disposed at the end of the guide tube and engages the fetal scalp. A second electrode for monitoring is also disposed at the end of the guide assembly in the holder of the spiral retaining coil. The two electrodes are connected to signal leads which are externally connected to the monitoring device. After connection with the fetal scalp, the guide assembly detaches from the electrode holder and the signal leads, thereby leaving just the lead wires and electrodes in place. The invention does not show the use of LEDs or other light measurement means for the electrodes.
The Helfer et al U.S. Pat. No. 4,437,467 discloses an apparatus for monitoring fetal heartbeat and the like, and this apparatus is very similar to that of the Murphy patent, in that there is an elongated guide tube which has an electrode coil assembly at one end for engaging the fetal scalp. The opposite end of the guide assembly has a handle that rotates the electrode coil assembly such that the coiled electrode assembly will pierce the fetal scalp. Furthermore, the use of light means for the electrodes is not disclosed.
The Tan U.S. Pat. No. 5,127,407, discloses an epidural oxygen sensor. In one embodiment, the sensor is comprised of a thin rubber-like probe that is inserted into the skull of the fetus. The sensor utilizes LEDs and a photo detector to monitor the oxygen saturation of blood within the skull of the fetus. The sensor is inserted through a burr hole drilled in the skull separately. In another embodiment, the LEDs and photo detector are situated within a hollow bone screw. The sensor in this embodiment is then screwed into the skull of the fetus at the desired depth. Upon receiving light sensors from the LEDs, the photo detector converts these signals to electrical signals for processing by external equipment.
The Hochberg et al U.S. Pat. No. 4,658,825 discloses a spiral probe for simultaneous electrical and chemical monitoring of a fetus, which includes a spiral-shaped needle which serves as an EKG electrode, and also houses a fiber optic pH probe. This combined probe is made of a soft material such as silicon plastic. An elongated guide tube is used to secure the probe in place and is then removed once it is secured to fetus. The fiber optic pH probe utilizes a separate light source and light sensor for monitoring the fetal pH. The light sensor here serves the same function as the photo detectors explained above. The needle electrode for EKG monitoring is also connected to an external monitoring device.
The Yount U.S. Pat. No. 4,968,137, and Neese et al. U.S. Pat. No. 5,046,965, relate generally to fetal monitoring.
Other literature articles of interest include the following:
1. John W. Severinghaus, M.D. et al., "Recent Developments in Pulse Oximetry," Anesthesiology, Vol. 76, pp. 1018-1038, 1992. PA0 2. Jason O. Gardosi et al., "Adaptation of Pulse Oximetry for Fetal Monitoring During Labor," The Lancet, Vol. 337, pp. 1265-1267, May 25, 1991. PA0 3. William W. Hay, Jr., M.D. et al., "Pulse Oximetry in Neonatal Medicine," Clinics in Perinatology, Vol. 18, No. 3, pp. 441-472, September 1991. PA0 4. Watson A. Bowes III, M.D. et al., "Pulse Oximetry: A Review of the Theory, Accuracy and Clinical Applications," Obstetrics Gynecology,Vol 74, pp 541-546, 1989.
However, the most troublesome aspect of all this prior art apparatus is the risk of trauma to the fetus. Clamping a fold of tissue for extended periods of time with sufficient pressure to maintain stability of the probe would likely result in necrosis of the clamped skin fold, not to mention the risk of burns from the heat generated from the light sources over long periods of time on particularly vulnerable fetal skin surfaces.
Therefore, it is desirable to be able to provide an oximetric apparatus which is convenient to use and practical to apply to the fetal head during early as well as active phases of labor. The apparatus must minimize trauma to the fetus and provide continuous measurements, including the arterial oxygenation levels in the fetus, as well as conventional FHR and EKG activity. Heretofore, although many devices for fetal monitoring have been proposed in the prior art, they have not been successful to date in overcoming the numerous problems recognized in the art, and have failed to provide the critical measurements which accurately reflect fetal well-being during labor.